The present invention relates to an automatic focussing system for driving an objective lens provided in a still camera, television camera and the like into an in-focussed position so that a focussed image can be formed on a film or an image pick-up plane.
In one known automatic focussing system, a sharpness, i.e. contrast of an image formed on the film surface or the pick-up plane is detected and the detected signal is processed in accordance with a predetermined estimation function to obtain a focussing error signal. In another known system, a pair of photo detectors are arranged in front and behind a predetermined focal plane and output signals from the detectors are processed in accordance with a given estimation function to derive the focus information. However, in these known systems, since the photo detectors are arranged stationarily, the focussing error signal cannot be obtained accurately when the objective lens is deviated from a given position to a large extent.